Presentation on theme: "Organic Chemistry Lab II, Spring 2009"— Presentation transcript:
1 Organic Chemistry Lab II, Spring 2009 Synthesis of Alkenes 1: Miniscale Synthesis of Alkenes Via the Acid-catalyzed Dehydration of 3,3-dimethyl-2-butanolOrganic Chemistry Lab II, Spring 2009Dr. MilkevitchMarch 1 & 3, 2010
2 Elimination Reactions Today: Conduct an elimination reactionOne of the fundamental reactions of organic compoundsWhat you will learn/observe:Review elimination reactionsLearn how to synthesize an alkeneAcid catalyzed dehydration of an alcoholLook at the products formedSee which product will predominate
3 Elimination Reactions Let’s Review:Elimination reaction: a fundamental organic reactionTwo species are eliminated from a substrateElimination mean’s they are gone, gone, goneNOT a substitutionBasic mechanism:
4 Another Kind of Elimination Reaction A dehydration reactionMultiple step reactionLoss of water, forming a carbocationAlkene formation resultsMechanism: E1 mechanismE1 elimination of a protonated alcoholReaction of alcoholsAcid catalyzedForms an alkene and water
5 General Mechanism: Acid-Catalyzed Dehydration of an Alcohol Step 1: Protonation of hydroxyl groupStep 2: Ionization to carbocation
10 Products: Don’t Forget Formed from secondary carbocation
11 Which Product Prevails? More substituted alkene predominatesCalled Zaitsev’s Rule
12 Procedure Construct your simple distillation apparatus Use a 25 ml RB flaskPlace 6 ml of 3,3-dimethyl-2-butanol into this flaskInclude a magnetic stir barAdd 6 ml of 85% phosphoric acid to this flaskPlace on apparatusStart stirring and heatingCollect distillate in a graduated cylinderIn an ice bathDistillate should be cloudyDon’t let the temperature rise above 75 deg C!!
13 Procedure II Complete distillation Turn off heat, let apparatus cool Disassemble distillation apparatusClean with acetoneSet aside to dryTransfer distillate to a small separatory funnelDrain lower aqueous layer into a beakerWash organic layer with 20 ml of saturated salt solnDrain off lower aqueous layerPour off organic layer into a 25 ml erlenmeyer flaskDry with anhydrous magnesium sulfate
14 Procedure III Gravity filter into a clean, dry 10 ml RB flask Reassemble distillation apparatusDistill againCool receiving flask in an ice bathWeigh a small vial, record weight of this flaskWhen distillation is done, transfer distillate to this vialReweigh the distillate in the vialDo a GC analysisDetermine the % of your alkenesDetermine the % of your starting material (3,3-dimethyl-2-butanol)Accomplish a IR spectrum
15 Your Report Formal Report Required Make sure to include the mechanism of the reaction in your report (separate section)Results Section:Make sure to state all resultsProve that you made the alkenesInclude GC chromatogram (annotated)Determine % of your alkenesIR spectrum, annotated (if completed)Conclusions section:Was the experiment successful?How much of the alkenes are produced?Any leftover 3,3-dimethyl-2-butanol?If unsuccessful, any ideas why?